Dynamic range is a very important parameter of any imaging system. Human vision has the capability to see details across a wide illumination range in a single scene, and is reported to exhibit around 200 dB of dynamic range. Scenes in excess of 100 dB are not uncommon in everyday situations. Consequently, designers of image sensors are continuously looking for ways to increase dynamic range.
In the field of CMOS image sensors, sensors having a log characteristic are used for image scenes having a high dynamic range. In a logarithmic mode the pixel voltage is continuously available to the outside world and no integration time is used. The photocurrent that is induced flows through one or more MOS transistors and sets up a gate-source voltage that is proportional to the logarithm of the photocurrent. This is exemplified in FIG. 1 where the gate-source voltage appears across M2. Since the photocurrent is very small, the MOS device(s) will operate in a sub-threshold, and thus the voltage varies logarithmically with the photocurrent. The voltage is read out by source follower circuitry. Around six decades of light can be captured in the logarithmic mode.
Due to the small devices used in the pixels, a high degree of mismatch results from process variations, which produces fixed pattern noise (FPN) across the array. Logarithmic sensors cannot use double sampling (in its conventional form) for removing mismatch since this technique only removes the variation of device M1 and does not alter the effect of device M2.
Another disadvantage of the logarithmic arrangement is a slow response time for low light levels. Increased photocurrent for a given light level can be accomplished by increasing the size of the light sensing element, but this is not desirable since the cost for a given resolution will increase accordingly.
The prior art discloses designs intended to combine features of linear and logarithmic responses, for example, U.S. Pat. No. 6,323,479 to Hynecek et al., and the article to Tu et al. titled “CMOS Active Pixel Sensor with Combined Linear and Logarithmic Mode Operation”, IEEE Canada conference on Electrical and Computer Engineering 1998, vol. 2, pp 754-757, 1998. However, these prior art proposals do not address the FPN and slow response problems.